304-STAINLESS-STEEL OXIDATION IN SULFATE AND SULFATE PLUS BICARBONATESOLUTIONS

The electrochemical characterization of 304 stainless steel in 0.1-0.5 M Na2SO4 and Na2SO4 + NaHCO3 aqueous solutions at pH similar to 8 was done in combination with SEM surface analysis. Passivation of the sur face film without any pitting events is observed for the current-poten tial and current-time experiments, and no anodic current spikes, which are associated with depassivation events, are distinguishable above t he background current level. However, SEM pictures of an electrode sur face polarized at potentials above 0.4 V show microscopic pit nucleati on. Even when the metal may be regarded as passive from a current-time or current-potential characteristics, passivity is not stable and loc alized film breakdown can still occur. The events are ascribed to pit nucleation at the active inclusion sites. The pits do not grow even in to the metastable state but die through repassivation by metal salt pr ecipitation immediately after birth. The effect is ascribed to the sol ubility of metal salts with the electrolyte produced by dissolution in the nucleation sites. The results show that pit nucleation and pit gr owth are two distinct processes. The importance of solution compositio n and the protective effect of bicarbonate ions is also discussed.